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, GUIDELINES FOR DETERMINING THE PROBABILITY OF CAUSAnON UNDER

cC THE U.S. ENERGY EMPLOYEES OCCUPAnONAL ILLNESS COMPENSATION

PROGRAM ACT OF 2000

M. Scbubauer-Berigan, L. J. Elliott, T. Katz, J. Neton

The National Institute for Occupational Safety and Health (NIOSH), United States of America,

e-mail: liel@cdc.gov

Abstract

This paper presents the probability of causation guidelines [1] implemented under select

provisions of the Energy Employees Occupational Illness Compensation Program Act of 2000

("EEOICPA" or "Act") [2]. These guidelines are used to determine whether an individual with

cancer shall be found, "at least as likely as not," to have sustained that cancer from exposure to

ionizing radiation in the performance of duty for nuclear weapons production programs of the

U.S. Department of Energy (DOE) and its predecessor agencies. These guidelines were

developed by the U.S. Department of Health and Human Services' (ffifS) National Institute for

Occupational Safety and Health (NIOSH) and are applied by the U.S. Department of Labor

(DOL), which is responsible for detemIining whether to award compensation to individuals

seeking federal compensation under the Act.

1. Background

Under EEOICP A, a covered employee with cancer is eligible for compensation only if DOL

determines that the cancer was "at least as likely as not" (a 50% or greater probability) caused

by radiation doses incurred in the line of duty while working for DOE and/or an Atomic

Weapons Employer. The HHS, NIOSH guidelines on probability of causation provide DOL

with the procedure to make these detemIinations, and specify the information DOL will use.

EEOICP A required the guidelines be based on the radiation dose received by the employee,

incorporating dose reconstruction methods established by HHS, NIOSH and perfomIed by

NIOSH. Also, risk determinations were required to be based on the upper 99 percent

"confidence interval" (credibility limit) of the probability of causation found in

radioepidemiological tables published under section 7(b) of the Orphan Drug Act, and as such

tables may be updated.

,

2. Discussion

Probability of causation as used in compensation programs is defined as an estimate of the

probability or likelihood that the illness of an individual member of an exposed group was

caused by the health hazard. Other terms for this concept include "assigned share" and

"attributable risk percent." For these guidelines, the potential hazard is ionizing radiation to

which U.S. nuclear weapons workers were exposed in the line of duty; the illnesses are specific

types of cancer. The probability of causation (PC) is calculated as the risk of cancer attributable

to radiation exposure (RadRisk) divided by the sum of the baseline risk of cancer to the general

population (BasRisk) plus the risk attributable to the radiation exposure, then multiplied by 100

percent, as follows:

RadRisk xlOO% = PC

RadRisk + BasRisk

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This calculation provides a percentage estimate between 0 and 100 percent, where 0 would

mean 0 likelihood that radiation caused the cancer and 100 would mean 100 percent probability

that radiation caused the cancer.

In 1985, the U.S. National Institutes of Health developed a set of radioepidemiological tables

[3]. The tables serve as a reference tool providing probability of causation estimates for

individuals with cancer who were exposed to ionizing radiation. Use of the tables requires

information about the person's radiation dose, gender, age of exposure, date of cancer diagnosis

and other relevant factors. The tables have been used by the U.S. Department of Vet~

Affairs (DV A) to make compensation decisions for veterans with cancer who were exposed in

the line of duty to radiation from atomic weapon detonations. The primary source of data for

the 1985 tables is research on cancer-related deaths occurring among Japanese atomic bomb

survivors from World War II. The 1985 tables have been updated by the National Cancer

Institute (NCI) and the Centres for Disease Control and Prevention to incorporate progress in

research on the relationship between radiation and cancer risk as applicable for use in the DV A

compensation program [4]. NIOSH has incorporated further modifications which are applicable

for use of the radioepidemiological tables under EEOICP A [5].

A major scientific change achieved by these updates is the use of risk models developed from

data on the occUlTence of cancers (cases of illness) rather than the occurrence of cancer deaths

among Japanese atomic bomb survivors. The risk models are further improved by being based

on more current data as well. Many more cancers have been modelled in the revised

radioepidemiological tables. The new risk models also take into account factors that modify the

effect of radiation on cancer, related to the type of radiation dose, the amount of dose, and the

timing of the dose.

A major technological change accompanying this update, which represents both a scientific and

a practical improvement, is the production of a computer program for calculating probability of

causation. This computer program, named the Interactive RadioEpidemiological Program

(IREP), allows the user to apply the risk models directly to data on an individual employee.

This makes it possible to calculate probability of causation using better quantitative methods

than could be incorporated into printed tables. In particular, IREP allows the user to take into

account uncertainty concerning the information being used to calculate probability of causation.

There typically is uncertainty about the radiation dose levels to which a person has been

exposed, as well as uncertainty in the science relating levels of dose received to levels of cancer

risk observed in study populations. Accounting for uncertainty is important because it can have

a large effect on the probability of causation estimates. The mIS, NIOSH guidelines and the

NIOSH-IREP, as required by EEOICP A, use the upper 99 percent credibility limit to detennine

whether the cancers of employees are at least as likely as not caused by their radiation doses.

This minimizes the possibility of denying compensation to claimants under EEOICP A for those

employees with cancers likely to have been caused by radiation exposures.

The risk models developed by NCI and CDC for the NCI-IREP provide the primary basis for

developing guidelines for estimating probability of causation under EEOICP A. They directly

address 33 cancers and most types of radiation exposure relevant to employees covered by

EEOICPA. These models take into account the employee's cancer type, year of birth, year of

cancer diagnosis, and exposure infoInlation such as years of exposure, as well as the dose

received from gamma radiation, x rays, alpha radiation, beta radiation, and neutrons during each

year. The risk model for lung cancer takes into account smoking history as well. None of the

risk models explicitly accounts for exposure to other occupational, environmental, or dietary

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carcinogens. It was not feasible to develop models accounting for these factors due to limited

information from existing research studies. Moreover, HHS and DOL could not consistently or

efficiently obtain the data required to make use of such models.

IREP models do not specifically include cancers as defined in their early stages: carcinoma in

situ (CIS). These lesions are becoming more frequently diagnosed, as the use of cancer

screening tools, such as mammography, have increased in the general population. The risk

factors and treatment for CIS are frequently similar to those for malignant neoplasms, and, while

controversial, there is growing evidence that CIS represents the earliest detectable phase of

malignancy. Therefore, for determining compensation under EEO ICP ~ CIS will be treated as a

malignant neoplasm of the specified site.

Cancers identified by their secondary sites (sites to which a malignant cancer has spread), when

the primary site is unknown, raise another issue for the application of IREP. This situation will

most commonly arise when death certificate infomlation is the primary source of a cancer

diagnosis. It is accepted in medicine that cancer-causing agents such as ionizing radiation

produce primary cancers. This means, in a case in which the primary site of cancer is unknown,

the primary site must be established by inference to estimate probability of causation.

mIS has established such assignments in these guidelines, based on an evaluation of the

relationship between primary and secondary cancer sites using the National Centre for Health

Statistics (NCHS) Mortality Database for years 1995-1997. Because national cancer incidence

databases (e.g., the National Cancer Institute's Surveillance, Epidemiology and End Results

program) do not contain information about sites of metastasis, the NCHS database is the best

available data source at this time to assign the primary site(s) most likely to have caused the

spread of cancer to a known secondary site. For each secondary cancer, the set of primary

cancers producing approximately 75% of that secondary cancer among the U.S. population was

identified (males and females were considered separately). The final assignment of a primary

cancer site for an individual claim will be determined by DOL on a case-by-case basis, as the

site among possible primary sites which results in the highest probability of causation estimate.

Claimants diagnosed with two or more primary cancers also raise a special issue for determining

probability of causation. Even under the assumption that the biological mechanisms by which

each cancer is caused are unrelated, uncertainty estimates about the level of radiation delivered

to each cancer site will be related. The consequence has simple but important implications;

instead of detennining the probability that each cancer was caused by radiation, DOL perfonns

an additional statistical procedure using IREP to determine the probability of causation for e~h

cancer and then the probability for both cancers combined. This will be important to the

claimant because it will determine a higher probability of causation than would be determined

for either cancer individually.

Some employees covered by EEOICP A were substantially exposed to radon and other sources

of high linear energy transfer (LET) radiation. This type of radiation exposure has unique

properties affecting cancer risk, specifically an inverse dose-rate effect for high-LET radiation

exposures. This effect means at any particular dose level, especially higher dose levels, a dose

of high LET radiation incurred gradually over time is more likely to cause cancer than the same

total dose incUn"ed quickly or at once. A substantial body of research supports this finding,

including studies of uranium miners, patients exposed to bone-seeking radium alpha particles,

and research on the cancer effects of high LET radiation in animals. Because high-LET

radiation is an important type of radiation exposure among employees covered by EEOICP A,

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NIOSH has modified IREP to include uncertainty associated with the assumption of an inverse

dose-rate effect for these exposures. Also, using epidemiologic evidence on the lung

carcinogenicity of radon exposures, a lung cancer model for radon exposures is incorporated

into the NIOSH-IREP. The data source for this model is the analysis conducted by the federal

Radiation Exposure Compensation Act Committee.

The DOE workforce has also been exposed to various types of neutron energies, which is

frequently documented in the worker's dosimetry records. The relative biological effectiveness

(RBE) of radiation exposure, a factor in cancer risk models that accounts for the differing level

of cancer risk associated with different forms of radiation, varies as a function of neutron

energy. This variation in RBE related to differing neutron energy is accounted for in the NIOSH

version of IREP.

References

[l.}

[2]

[3]

[4]

[5]

"Probability of Causation Guidelines Under the Energy Employees Occupational lllness

Compensation Program Act of 2000" proposed for promulgation at 42 CFR 81 (66 FR

50967), October 5, (2001).

The Energy Employees Occupational lllness Compensation Program Act, Public Law

106-398, 114 Stat. 1654, 1654A-1231 (October 30, 2000), enacted as Title XXXVI of the

Floyd D. Spence National Defense Authorization Act for Fiscal Year 2001.

National Institutes of Health (NllI). Report of the National Institutes of Health Ad Hoc

Working Group to Develop Radioepidemiological Tables. Nlli Publication No. 85-2748.

(1985) 355 pp.

National Cancer Institute (NCI). Draft Report of the NCI-CDC Working Group to Revise

the 1985 Nlli Radioepidemiological Tables (2000).

NIOSH-Interactive RadioEpidemiological Program Documentation - DRAFT, DilliS,

National Institute for Occupational Safety and Health, Cincinnati, Ohio (2001) 63 pp.

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